The fluid passages in plate-fin heat exchangers are defined by partitions of a metal which has a satisfactorily high coefficient of heat transfer, so that when a high temperature fluid is passed through some passages and low temperature fluid is passed through further passages which are adjacent thereto, there results a cooling of the originally high temperature fluid, by heat conduction through the thickness of the partitions into the cool fluid. Efficiency of heat exchange is boosted by inclusion in the fluid flow passages of so-called "fins", which may in fact be corrugated members, dimples, grooves, protuberances, baffles or other turbulence promoters, instead of fins as such.
Plate-fin heat exchangers offer significant advantages over shell-tube heat exchangers in terms of weight, space, thermal efficiency and the ability to handle several process streams--i.e. several streams of heat exchange media--at once. However, most current plate-fin heat exchanger technology is centred on a brazed matrix construction using aluminium components and is therefore limited to low pressure and low temperature operation. Even using other materials, such as stainless steel, operational pressure limits (say, 80-90 bar) apply because of brazing as the method of fabrication.
Our prior patent applications EP90308923.3 and GB9012618.6 disclose alternative ways of manufacturing plate-fin heat exchanger elements which help to avoid the above problems and allow greater flexibility in their design. Among other things, they describe a method of manufacturing heat exchange plate elements in which metal (e.g. titanium or stainless steel) sheets are stacked together and selectively diffusion bonded to each other before being superplastically deformed to a final hollow shape defining internal passages, which can incorporate integrally formed "fins". Use of superplastic deformation in the manufacturing process enables the generation of high volume fractions of hollowness in a heat exchanger element. The result is a high integrity, low weight heat exchanger element. For example, use of titanium alloy materials to produce heat exchanger elements by the diffusion bonding and superplastic forming route enables their operation at pressures in excess of 200 bar and at temperatures up to 300.degree. C., whereas stainless steel materials enable even better performance.